It is well known to cure inks on a substrate by application of ultra-violet radiation from one or more medium-pressure mercury vapor ultra-violet lamps. It is also well known to provide each lamp in an assembly with a reflector which includes a reflective surface partly surrounding the lamp for reflecting radiation therefrom onto the substrate. The reflective surface has a concave profile which is commonly elliptical or parabolic, the lamp being mounted on the symmetrical center line of the profile and adjacent the apex.
The reflector increases the intensity of the radiation received by the curable material. The penetration of the radiation into the material is an important factor in curing and, while penetration varies with different colors and materials, the higher the intensity the better the penetration.
A problem which arises with known arrangements is that part of the radiation is reflected back onto the lamp itself, which reduces the amount of radiation energy available for curing and leads to heating of the lamp which can adversely affect lamp operation and increase the already large amount of heat given off by the assembly, and which may cause warping and distortion of the coating and/or the substrate.
This problem has been recognized in French Patent 2334966 which describes a reflector in the form of two half-shells, each of which is pivotal about a longitudinal axis within the cavity to the sides of the symmetrical center line thereof. The French Patent proposes deforming the top region of the reflector to give it, externally, a generally concave shape across the width of the lamp by bending the top edge of each half-shell down towards the lamp.
The apparatus disclosed in French Patent 2334966 has disadvantages as a result of its basic form in that a complicated system will be necessary to achieve the desired pivoting action and space has to be provided to accommodate the half-shell pivoting which is inconsistent with the current industry desire for smaller curing assemblies. Cooling of the half-shells will be difficult, again because of the need to accommodate the pivoting action. Problems will also arise as a result of the solution proposed in the French patent to the problem of lamp self-heating. The distortion of the reflector towards the lamp will lead to excessive heating of the distorted portion and will make cooling of the adjacent region of the lamp much more difficult.
The efficient and effective cooling of lamp assemblies has been a constant problem which has become even more important as ever increasing lamp powers have been employed to give faster curing such that substrate speeds can be increased. For example, at the date of the French patent, 1975, maximum lamp powers were only in the region of 250 Watts per inch (100 Watts per cm). Lamp powers of 200-400 Watts per inch (80-160 Watts per cm) are now common and lamps of even higher powers, 500-600 Watts per inch (200-240 Watts per cm) are increasingly being used. Furthermore, the advantages of UV curing, including cleanness and quality, have led to a demand for curing systems capable of operating with a wide variety of substrates, including substrates which are very vulnerable to heat damage.
Earlier assemblies were generally cooled by air alone. In the first air-cooled systems, air was extracted from within the reflector through one or more openings provided above the lamp to draw out the heat. In later systems, cooling air was blown into the assembly and onto the lamp, again through openings located adjacent the lamp.
Increasing cooling requirements due to higher lamp powers led to the use of water cooling alone or in conjunction with air cooling. The cooling water is fed through tubes attached to or integrally formed in the reflector.
GB Patent Application 2336895 discloses a UV dryer for drying printing inks and the like provided with a pair of plates between the reflector and the substrate that are cooled with air or water. A heat sink cooled by water is positioned below the substrate. The dryer itself is cooled by air which is guided to flow over the inner and outer surfaces of the reflector into the upper part of the housing in which the reflector is contained. Air which flows over the inner surfaces of the reflector passes through a gap provided at the apex of the reflector and joins air which has passed over the outer surfaces of the reflector.
The UV lamp is mounted on a carriage which is slidable longitudinally off the housing into and out of the housing for lamp replacement purposes. The carriage includes electrical connector components which mate with corresponding connecting components located within the housing when the carriage is slid fully into the housing.
A significant drawback of water cooled systems is the cost of the water cooling equipment. A further drawback is the need to provide a closed water circuit while still allowing access to the lamp assembly to allow replacement of the lamp.
As a result, 80% of commercially available systems are air cooled. As lamp powers increase, ever more efficient and effective air cooling systems are required to keep temperatures within acceptable limits, not only to prevent damage to the substrate, but also to prevent harm to adjacent equipment and to operators.
As discussed above, there is a current industry desire for smaller curing assemblies. In addition there is a desire for curing assemblies which allow easy lamp replacement without the need to remove the lamp assembly from the curing assembly.